[lucet-wasi] increase default memory limits
[lucet-wasi] increase default memory limits

- Heap limits are set to maximum for a 32-bit address space + 32-bit guard size
- Stack size is set to 8MiB to match Linux's default
- Globals size is calculated from the module

fastly/lucet,fastly/lucet,fastly/lucet,fastly/lucet,fastly/lucet